Rereading DnK, it was obvious Rui wasn't gonna win, Sasuga always gunned for a Hina endgame, but botched the execution. Rui was a plot device to prevent Natsuo from returning with Hina because she was basically his maturity mirror and Natsuo's final step into adulthood wasn't settling down with Rui because of having left her preggo, it was taking responsibility for Hina's feelings.
DnK main motif was what does it mean to be an adult? And sure as shit it wasn't about wanting to rail your nbr oneechan, neither your nbr imouto, but to learn to communicate and compliment with your couple, and he sucked dick at both of that during tge whole run.